1. FIELD OF THE INVENTION
The invention is a process ,involving temperatures between room temperature and 100.degree. C., for coating a metal substrate with hydroxyapatite.
2. DESCRIPTION OF THE PRIOR ART
One method of fixation for orthopedic and endosseous implants is by bony ingrowth into the pores of a "porous coat" on the metallic surface. Some improvement in this fixation is achieved by plasma-spraying heat-treated hydroxyapatite onto the porous coat of the implant. Hydroxyapatite, containing a small amount of carbonate, is the principal inorganic component of bones and teeth. The high temperature involved in the plasma-spraying process, however, causes physical and chemical modifications of the hydroxyapatite powder and also some sintering of the underlying metal porous coat.
One cold-temperature method is to apply hydroxyapatite to a metal by electrophoresis from a suspension ofhydroxyapatite in an organic solvent. The resulting coating, however, significantly loosens and dissolves when exposed to water.
A hybrid cold/hot method is to electrodeposit tricalcium phosphate hydrate and then sinter the coating. This heat treatment, which produces substantial conversion of the coating to hydroxyapatite, also sinters to some degree an underlying porous metal coat.
The point of departure for this invention is a method of electrodepositing brushite from a solution of calcium phosphate monobasic. When a metal with a porous coat is the substrate, however, the brushite crystals are too easily abraded off for application as a coating for an implant. Another limitation is that brushite is not currently permitted as an implant coating.